The present invention is related broadly to home appliances that employ a heat-generating apparatus and, more particularly, to home appliances that use gas burners as a controlled heat source.
Home appliances such as ranges and cooktops may use gas burners as a source of heat for cooking. Cooking appliances that may employ gas burners include freestanding ranges that include an oven and a cooktop as well as built-in, stand-alone, wall-mounted ovens. With respect to the present invention, references herein to ranges and built-in ovens may be used interchangablely and both may act as a platform for gas burner use. In addition, while the present application focuses on ranges and ovens, the invention described herein may have applicability with other appliances employing heat such as laundry dryers and the like.
Gas burners utilize a gas supply such as natural gas or propane mixed with air to provide a combustible gas/air mixture for ignition by a resistive heater element. A user-controlled valve throttles the amount of gas available for the burner to thereby control the amount of heat energy applied by the burner flame to an oven cavity to raise the temperature within the cavity to a predetermined level for cooking.
In general, a gas-fired oven burner operates when a user first starts the oven or when a hot oven drops below a predetermined temperature. There, a user control or a thermostat switches power to an igniter and a gas valve circuit which are connected in series. It should be noted that the term “igniter” may also be presented as “ignitor”. Both spellings are valid and describe the same structure. As power flows through the igniter the current draw causes the igniter to produce heat. The igniter includes a resistive heater element joined to a base, usually ceramic, for mounting and connection to a power source. A heat shield or shroud keeps wiring and other undesirable matter away from the heater element.
Once the igniter draws a specific amount of current or achieves a pre-determined temperature; a gas valve opens to allow gas flow to the oven burner where the glowing hot igniter ignites the gas. Once the set temperature is achieved, the control stops all power to the ignition circuit which causes the igniter to dim and the oven gas valve to close thereby cutting off the burner flame. Cycling on and off continues in order to maintain the desired cooking temperature within the oven cavity.
Once the igniter is activated, it rapidly heats to glowing and the gas should light off quickly. However, with conventional shrouds such is not always the case.
As seen in FIGS. 7A and 8A, a prior shroud 100 includes a shroud body 102 that conforms generally to the base of the igniter and partially surrounds the heater element. The body 102 includes vented walls 104 having openings 106 formed therein to allow heat to escape from the igniter while still providing igniter protection. With reference to FIG. 8A, the heater element 110 is located generally centrally within the shroud 100 and partially surrounded by the vented walls 104. The entire assembly 100 is in operational communication with a gas pipe 32 to produce a flame F. It should be noted that a flame F is shown in FIG. 8A and FIG. 8B for clarity. It will be understood that gas is emitted from the pipe prior to ignition.
As the heater element 110 is heated, convection heat is emitted, as illustrated in FIG. 8A by rings C. Given the configuration of the shroud 100, air is effectively pushed away from the heater element 110 by convection action as the heat from the heater element 110 causes a general airflow away from the heater element 110. This convection flow can also cause the gas from the gas pipe 32 to be blown away from the heater element 110 thereby directing the gas into a region of the temperature field created by the heating element 110 that is at a lower temperature than areas closer to the heater element 110, and tending to disburse the gas, causing a lean gas/air mixture. Accordingly, lighting gas ignition for flame F production is delayed until the outer reaches of the temperature field created by the heating element 110 are hot enough to ignite the gas. This time can vary among individual igniters, but has taken as long as eight (8) seconds.
It is generally desirable to expect gas ignition within four (4) seconds or less after initiation of the ignition process. In addition, the Canadian Standards Association requires ignition within four (4) seconds or less before an appliance can be listed for sale in Canada. Prompt ignition is required to prevent accumulation of gas within the oven cavity which could prove dangerous. In addition to the initial heating of the oven, an unduly long wait for gas ignition can affect oven temperature stability and control, which can have a detrimental effect on appliance efficiency.
Accordingly, there exists a need for a gas igniter to counter the tendency of convection heat to move gas away from the igniter. There also exists a need to incorporate such structure into the existing structure of the burner assembly and more particularly, in an igniter shroud.